In the field of multi-cylinder engines having a plurality of cylinders, there has heretofore been known a technique, so-called “reduced-cylinder operation” in which a part of the cylinders is deactivated by stopping combustion therein.
During the reduced-cylinder operation, the number of activated cylinders, i.e., the number of output cylinders is reduced, thereby possibly leading to a decrease in power output of the entire engine. Therefore, it is common practice to execute, during the reduced-cylinder operation, a control for increasing an amount of air (intake air amount) to be taken into each of the activated cylinders so as to increase power output of the activated cylinders.
In this case, however, the intake air amount is increased, with a certain delay. Thus, even when, the control for increasing the intake air amount is executed at the time of switching from the all-cylinder operation to the reduced-cylinder operation, the intake air amount does not instantaneously increase, thereby leading to lowering of engine power output i.e. occurrence of torque shock at the time of switching.
In this regard, Patent Literature 1 discloses an apparatus configured to, at the time of switching from an all-cylinder operation to a reduced-cylinder, operation, shift a throttle valve provided in an intake passage communicating with each of the cylinders toward its fully-open side so that the opening angle of the throttle valve comes close to the opening angle during the reduced-cylinder operation, before combustion of a part of the cylinders is stopped so as to increase the amount of air (intake air amount) to be taken into all the cylinders, and then to stop combustion of a part of the cylinders.
In the apparatus disclosed in Patent Literature 1, the amount of air to be taken into each of the cylinders increases before combustion of a part of the cylinders is stopped. This makes it possible to suppress shortage of the intake air amount of the activated cylinders at the time of stopping combustion of the cylinders, namely, at the time of start of the reduced-cylinder operation. However, simply increasing the intake air amount of each of the cylinders before start of the reduced-cylinder operation, namely, in a state that combustion is performed in all the cylinders may increase power output of the entire engine before start of the reduced-cylinder operation. This may also cause torque shock.